<!DOCTYPE html>
<html>

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>placeholder</title>
    <style>
        input::placeholder {
            color: darkred;
        }
    </style>
</head>

<body>

    <input type="email" placeholder="请输入您的学号" id="email" size="25"/>
</body>

</html>